1. Field of the Invention
The present invention relates to a damping force control type of hydraulic shock absorber for use in a suspension system of a vehicle, for example, an automobile.
2. Description of the Prior Art
Hydraulic shock absorbers for use in a suspension system of an automobile or other vehicle include damping force control type of hydraulic shock absorbers which are designed so that the level of a damping force can be controlled properly in accordance with the road surface conditions, vehicle running conditions, etc., thereby improving the ride quality and the steering stability.
Japanese Utility Model Application Public Disclosure (KOKAI) No. 58-70533 (1983) discloses one example of a conventional hydraulic shock absorber of the type described above. The disclosed hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein, and a piston with a piston rod connected thereto, which is slidably fitted in the cylinder to define two chambers in the cylinder. The two chambers communicate with each other through first and second hydraulic fluid passages. The first hydraulic fluid passage is provided with a first damping Force generating mechanism (including an orifice, a disk valve, etc. ) that generates a relatively large damping Force, while the second hydraulic fluid passage is provided with a second damping force generating mechanism that generates a relatively small damping force, together with a damping force control valve that opens and closes the second hydraulic fluid passage.
With the above arrangement, when the damping force control valve is open, the hydraulic fluid in the cylinder flows mainly through the second hydraulic fluid passage in response to the sliding movement of the piston caused by the extension and retraction of the piston rod, thereby generating a relatively small damping force during both the extension and retraction strokes. Thus, "soft" damping force characteristics are obtained. When the damping force control valve is closed, the hydraulic fluid in the cylinder flows through only the first hydraulic fluid passage in response to the sliding movement of the piston caused by the extension and retraction of the piston rod, thereby generating a relatively large damping force during both the extension and retraction strokes. Thus, "hard" damping force characteristics are obtained. In this way, the damping force characteristics can be switched by opening and closing the damping force control valve.
In the meantime, a suspension control system has been known wherein the damping force characteristics of the above-described damping force control type of hydraulic shock absorber are automatically switched in accordance with the road surface conditions, vehicle running conditions, etc. by using a controller and an actuator, thereby improving the ride quality and the steering stability.
In the suspension control system of the type described above, when the piston rod moves toward a predetermined neutral position, the damping force characteristics of the hydraulic shock absorber are made "hard", whereas, when the piston rod moves away from the neutral position, the damping force characteristics are made "soft", whereby it is possible to control the bouncing of the vehicle body effectively and hence Improve the ride quality.
However, when such bouncing control is effected by using the above-described conventional damping force control type of hydraulic shock absorber, some problems arise. That is, when different damping force characteristics are needed for the extension and retraction strokes of the piston rod, e.g., "hard" damping force characteristics during the extension stroke, and "soft" damping force characteristics during the retraction stroke, the damping force control valve must be opened and closed each time the extension and retraction strokes change from one to the other. However, it takes about 15 msec to 20 msec in general from the time when the controller outputs a switching signal upon detecting a change in the stroke direction of the piston rod until the damping force control valve is opened or closed by the actuator that is activated In response to the switching signal. Therefore, it is difficult to effect an adequate control during the actual running of the vehicle.